These core volume relationships are very empirical. The numbers refer to just
the core of the heat exchanger (fins and all) but exclude the inlet and
outlet tanks. As I noted in the relationship ratio, there is an allowance
made for the different efficiency of horizontal crossflow designs as opposed
to the vertical type. These are first order fits to the properties of
hundreds of different systems and are used for the initial setup. If
inadequate (because of available surface area or volume) then experiments
follow.

Frank Grunthaner